Differences between playing against the 4-3 and 3-4 are subtle but pronounced.
Despite the recent rise of the 3-3-5 and the 4-2-5, the primary defenses that most football teams will run are the 4-3 and the 3-4. While those designations tend to be a bit of a misnomer (especially against spread teams, where the D will be forced into its Nickel and Dime packages more often), they are quite important when determining the proper reads for a quarterback to make.
Now, reads will be different for every team, and should be adjusted to the playing style of each individual defense, but some trends have developed in how teams tend to read different defenses.